Implantable hearing aids entail the subcutaneous positioning of some or all of various hearing augmentation componentry on or within a patient's skull, typically at locations proximal to the mastoid process. In a semi-implantable hearing aid, a microphone, signal processor, and transmitter may be externally located to receive, process, and inductively transmit a processed audio signal to an implanted receiver. The processed audio signal is provided to an implanted transducer, which stimulates a component of the auditory system to produce/enhance the sensation of sound for the patient.
In one example of a semi-implantable hearing aid, the implanted receiver is located beneath the skin of a patient, e.g. within the mastoid process, while the external transmitter is included in a housing detachably connectable to the patient proximate to the implanted receiver. In this characterization, the housing is connected to the patient via an implanted magnet oriented to produce a mutual attraction with a magnet located in the external housing. The external housing is typically positioned adjacent the implanted magnet on the patient's skull, e.g. behind the ear. The mutual attraction of the magnets serves to hold the external housing in place such that a desirable separation exists between a transmitting coil of the external transmitter and a receiving coil of the implanted receiver.
The desired separation is somewhat determined by the thickness and properties of the patient's skin separating the two magnets. As will be appreciated, since magnetic attractive force falls off rapidly with distance, in some cases the external housing may not be held securely against the skin of the patient. This in turn results in an undesirable separation or distance between the external and implanted coils. One solution to this problem is to use a stronger magnet(s) to connect the external housing. As will further be appreciated, however, the resulting pressure from the mutual attraction of the magnets may impair the circulation of blood in the capillaries of the patient's skin. Thus, it is desirable that the pressure applied on any part of the patient's skin be minimized to the extent possible. This often results in a tradeoff between avoiding pressure that may impair circulation and achieving a desired level of magnetic attraction, e.g. such that the external component is not held so loosely that an undesired coil separation exists.
As will also be appreciated, a tether including a means such as a clip for attaching to the patient, e.g. the patient's hair, may also be included on the external housing portion of the hearing aid. The tether functions as a safety link between the external housing and the patient to protect the external housing from falling and becoming damaged in the event that the magnetic attraction is broken during use.